Storage batteries, which are widely used as power sources in a variety of applications, are typically classified into two general groups: lead storage batteries and alkaline storage batteries. Between the two groups, alkaline storage batteries tend to be more reliable, and can be made smaller and lighter. Small alkaline batteries are generally favored for portable electric appliances, while large alkaline batteries have been used mainly in conjunction with industrial equipment.
While some alkaline storage batteries use, for example, silver oxide or simply air for their positive electrode, more commonly the positive electrode is nickel. Nickel electrodes have been particularly popular since they were reconfigured from a pocket type to a sintered type, and became even more popular with the development of hermetic-sealing. Cadmium is most commonly used to form the negative electrode of alkaline storage batteries, however other materials, including zinc, iron, hydrogen, and the like have also been employed.
There is considerable commercial interest in storage batteries that have a higher energy density than batteries currently available. By "energy density" is meant the product of the battery capacity and the battery voltage per unit weight or volume, usually represented by Wh/kg or Wh/l. Toward achieving this goal, researchers have investigated nickel-hydrogen storage batteries which incorporate hydrogen storage electrodes. The alloys in these electrodes, or the hydride form of such alloys which form upon the absorption of hydrogen by the alloys, can absorb and desorb hydrogen in a reversible manner, and thus the alloys and the electrodes made from these alloys have come to be known as hydrogen storage alloys and hydrogen storage electrodes (or hydrogen storage alloy electrodes), respectively.
Batteries made with hydrogen storage electrodes have a larger theoretical capacity density compared to batteries formed with cadmium electrodes. By "capacity density" is meant the discharge capacity per unit weight or unit volume of an alloy, usually represented by mAh/g or mAh/cc. Also, batteries that employ hydrogen storage electrodes are not susceptible to the formation of dendrites, which is a problem with zinc electrodes that can cause a battery to short-circuit. These advantageous properties, as well as the promise of a longer cycle life and a reduction in the environmental concerns inherent in zinc and cadmium containing electrodes/batteries, have encouraged research into developing alloys suited for hydrogen storage electrodes, and particularly negative electrodes for alkaline storage batteries. By "cycle-life" is meant the ability of a battery to maintain a high discharge capacity after repeated charging and discharging cycles.
Prior art alloys for hydrogen storage electrodes include multi-element alloys such as those of either the Ti--Ni system, or the La (or Mm)--Ni system (where Mm is a misch metal). Multi-element alloys are typically prepared through either an arc melting process, an induction heating melting process, or some similar process.
The multi-element alloys of the Ti--Ni system are classified as an AB type (where A is La, Zr, Ti or an element with a similar affinity for hydrogen, and B is Ni, Mn, Cr or any other transition metal). When this type of alloy is used as the negative electrode in an alkaline storage battery, the electrode exhibits a relatively large discharge capacity during the initial charging and discharging cycles. However, electrodes comprising these alloys do not maintain their large discharge capacity after repeated charging and discharging cycles, i.e., do not have large saturation discharge capacities.
Another multi-element alloy is the La (or Mm)--Ni system, which is classified as an AB.sub.5 -type, where A and B are defined as above in relation to the AB type of alloy. These electrodes, and particularly electrodes made from the Mm--Ni system, have been put to commercial use as negative electrodes, but suffer from several disadvantage. For example, electrodes based on this class of alloy have a relatively small discharge capacity and an undesirably short cycle-life for a storage battery. In addition, the materials for these alloys are expensive. Therefore, it is desired to develop novel alloys from which hydrogen storage electrodes having a large discharge capacity and a long cycle-life can be made.
A Laves phase alloy of an AB.sub.2 -type has the potential to overcome many of the shortcomings of the multi-element alloys described above. AB.sub.2 -type alloys have two main crystal structures: a C15 face-centered cubic structure, as present in MgCu.sub.2, and a C14 hexagonal structure, as present in MgZn.sub.2. Electrodes for a storage battery formed from a Laves phase alloy of an AB.sub.2 -type have relatively high hydrogen storing capability and exhibit a high capacity and a long cycle-life. Alloys having a Laves phase of the AB.sub.2 -type are known in the art. See, e.g., U.S. Pat. No. 4,946,646 (AB.alpha. alloy), U.S. Pat. No. 5,149,383 (alloy comprising Zr.sub..alpha. Mn.sub..beta. M.sub..tau. Cr.sub..delta. Ni.sub..epsilon.), U.S. Pat. No. 5,205,985 (alloy comprising ZrMn.sub.w V.sub.x M.sub.y Ni.sub.z), and U.S. Pat. No. 5,096,667 (alloy comprising V, Ti, Zr, Ni and Cr).
By adjusting the composition in a Laves phase alloy comprising Zr, Mn, V, Cr and Ni, a hydrogen storage electrode having a discharge capacity of 350 mA-hr/g or above has been obtained (U.S. Pat. No. 5,149,383). In addition, by adjusting the composition in a Laves phase alloy comprising Zr, Mn, V, M and Ni (where M represents a member selected from the group consisting of Fe and Co), the discharge characteristics during the early charging and discharging cycles of a hydrogen storage electrode have been improved while maintaining the high capacity of the electrodes (U.S. Pat. No. 5,205,985).
When a Laves phase alloy of the AB.sub.2 -type is used as an electrode in a storage battery, it is possible to obtain a larger discharge capacity and a longer cycle-life compared with electrodes based on the multi-element alloys of the Ti--Ni system or the La (or Mm)--Ni system. However, further improvements in the performance of negative electrodes formed from Laves phase alloys of the AB.sub.2 -type are still desirable.
When a nickel-hydrogen storage battery is configured with a hydrogen storage electrode known in the prior art, the temperature of the storage battery will rise during rapid charging of the battery. The temperature rise is primarily due to heat generated by hydrogenation or similar reactions at the negative electrode. Therefore, and undesirably, the hydrogen equilibrium pressure of the hydrogen storage alloy in the negative electrode rises. Another disadvantage is that the gas pressure inside a battery vessel rises with increasing temperature, thereby inviting liquid to leak from the vessel, and causing a large decrease in the amount of the hydrogen stored in the alloy.
Thus, there is a great demand for a novel alloy which can be used to produce an electrode capable of maintaining the inner gas pressure of the battery at a low level, even at the high temperatures (about 80.degree. C.) present during the rapid charging cycle. There is also a demand for batteries which do not leak liquid, which maintain a high capacity of hydrogen storage, and have excellent initial discharge characteristics and saturation discharge capacities.